This invention relates to aircraft landing systems, and is more specifically directed to a radio landing system in which the position of an aircraft approaching a runway can be determined precisely, in terms of azimuth, range, and elevation relative to the runway. The invention is particularly concerned with a landing system which employs radio interferometry to guide the aircraft to a safe landing. The same or similar system can also guide the aircraft on the ground while taxiing to avoid ground collisions or running off the tarmac, runway, or taxiway. The system can operate with an unlimited number of aircraft simultaneously, and can provide all aircraft with position information as they perform the landing maneuver, and can do so without the aircraft experiencing radio interference with one another.
Currently under adverse visibility conditions, airports employ an Instrument Landing System (ILS) to guide instrumented aircraft to land. This system is often lacking in precision and reliability and requires visual means to complete the touchdown and roll out portions of the landing maneuver.
Landing systems have also been proposed which employ a number of radio antennas, arrayed in a geometrical configuration on the ground, to guide an aircraft during its landing maneuver under low visibility conditions. One such proposal involves a triangulation technique known as trilateralization to derive X,Y,Z coordinates of an aircraft. In this case the radio antennas are disposed about halfway down the runway and at a distance at opposite sides of the runway center line. A transponder aboard the plane answers a coded interrogation, and the return signal is received at each of the ground antennas, which then permit computation of the respective radii to the aircraft. This system, described in U.S. Pat. No. 3,564,543, requires a specially modulated square-wave signal, with pseudorandom noise coding to permit ranging.
Other aircraft landing systems include fan beam types with antennas located at each side of the runway emitting slanted signals. An example of such a system is described in U.S. Pat. No. 3,713,159. Other guidance systems are described in U.S. Pat. No. 4,017,862, and U.S. Pat. No. 3,789,646.